In the area of application of paper for sanitary or domestic use, such as toilet paper or all-purpose wipes, the production of sheets of paper made up of several plies of absorbent paper, such as of tissue paper, with a square meter surface measure weight ranging from 12 to 30 g/m2 provided with projections which are obtained by embossing, is a known technique.
Embossing in fact imparts bulk to the sheet and leads to improvement in absorption of liquids, feel, and softness. An attempt has been made to increase the absorptivity further by creating multilayer sheets obtained by combining two layers, also called plies, each made up of at least one of such embossed sheets.
Thus, a multilayer sheet is obtained that has mechanical characteristics such as specific resistance to traction as well as a specific absorptivity.
Two methods of embossing and assembly of such plies are currently applied, depending on the characteristics desired for the final product.
The first is known in this particular field under the designation of the “nested” design method. It consists first of embossing each of the plies separately so as to form on the surface projections which are generally substantially tapered in shape or are shaped as truncated pyramids. Adhesive is then applied to the top or apex of the projections of one of the plies, and the plies are arranged so that the surfaces present facing the projections, with the projections of one surface being opposite the projections of the other surface. Lastly, the plies are assembled so that the projections which have been glued nest within the projections of the other ply. Thus the two plies are connected by adhesive points between the tops of the projections of one ply and the non-embossed zones between the projections of the other ply. A structure is thus created, the voids of which made in this manner are suitable to ensure improved absorption for the sheet. In addition, the outer surfaces have a smooth and soft feel due to the hollow areas formed by the back of the projections. This technique is shown in U.S. Pat. No. 3,867,225.
The second assembly method is known in this field as “tip-to-tip” assembly. It differs from the preceding method in the relative arrangement of the two plies. After the latter have been separately embossed, they are placed one on top of the other so that the apexes of the projections coincide or so that at least a portion of the apexes coincide. The plies are connected to each other by the tops of the projections, tip to tip. This technique is shown in U.S. Pat. No. 3,414,459.
When the practical embodiments of these structures, whether of the nested or tip-to-tip type, are examined, it is found that the projections obtained during embossing are most often produced in such a way as to be distributed according to a pattern in which the projections are generally aligned uniformly according to one or several directions. One such design permits a homogenous sheet relative to its mechanical characteristics to be obtained.
However, in order to improve user satisfaction, it is important to increase the suppleness of the sheet.
The suppleness of the sheet can be defined as being its capacity to be deformed regularly when it is placed in the hand of a user, his fingers being stretched out in an extension of his palm and when the user then pulls back his fingers.
A sheet of paper which presents a high degree of suppleness is then going to be deformed and be curved or cambered in such a way as to remain more or less parallel to the form determined by the interior of the palm of the hand of the user.
A sheet of paper which presents a low degree of suppleness will be folded along one or several lines, each part of the sheet that extends itself between two folding lines remaining more or less flat.
A sheet of paper which has a high degree of suppleness is more convenient to use.
Generally speaking, an increase in suppleness of the sheet causes in particular a decrease in its mechanical characteristics.